1. Field of the Invention
The invention relates to liquid crystal display (LCD) devices, and in particular to cholesteric liquid crystal display (Ch-LCD) devices integrated with actuating devices.
2. Description of the Related Art
Liquid crystal display (LCD) devices have many advantages such as a smaller size, lighter weight and lower power consumption, and are applicable in a variety of electronic and communication devices including notebook computers, personal digital assistants (PDA), mobile phones and the like due to its lighter weight, thinner profile, and portability.
Conventional reflective memorable color liquid crystal display devices are widely applicable in electronic books, electronic papers, and the likes. After a conventional cholesteric liquid crystal display (Ch-LCD) is driven, arrangement of the cholesteric liquid crystal molecules cannot return to their initial arrangements, resulting in high reflection losses at bright state. Moreover, when display image of the Ch-LCD device is renewed, the cholesteric liquid crystal molecules have to be switched to a hemeotropic state, also referred as “H” state, the driving voltage of which exceeding about 50 volts.
The driving techniques of the conventional Ch-LCD device is achieved by voltage driving, i.e., change waveform and time frame of the driving signal to drive phase transition of the cholesteric liquid crystal. Voltage driving methods, however, cannot make the cholesteric liquid crystal molecules return to the original planar state (i.e., bright state), resulting in high reflection losses at bright state. The following description introduces several driving techniques for Ch-LCD devices.
U.S. Pat. No. 4,472,026, the entirety of which is hereby incorporated by reference, discloses a Ch-LCD device using thermal driving method to drive phase transition of the cholesteric liquid crystal. FIG. 1 is a cross section schematically showing a conventional Ch-LCD device. Electric field is exerted on the cholesteric liquid crystal molecules to change from horizontal arrangement to vertical arrangement. The cholesteric liquid crystal molecules are heated to change from vertical arrangement to horizontal arrangement. Referring to FIG. 1, a Ch-LCD device 10 includes an upper substrate 11 and a lower substrate 16. Electrodes 17a and 17b are disposed on the lower substrate 11, and a common electrode 15 is disposed on the upper substrate 16. Alignment layers 12, 18a, and 18b are disposed on the lower substrate 11, and an alignment 14 is disposed on the common electrode 15. A cholesteric liquid crystal layer 13 is interposed between the upper substrate 16 and the lower substrate 11. Conventional driving method for the CH-LCE is achieved by heating. Passive matrix electrodes are biased passing current therethrough. The cholesteric liquid crystal is transformed from a dark-state arrangement to a bright-state arrangement due to electrode resistive heating. Therefore, it is prevented that the cholesteric liquid crystal is driven to the “H” state under a high voltage. High reflection losses, however, are still inevitable.
U.S. Pat. No. 6,753,933, the entirety of which is hereby incorporated by reference, discloses a pixel chamber structure in a Ch-LCD device. Arrangement of the cholesteric liquid crystal molecules can be changed by exerting pressure on the pixel chamber. FIG. 2 is a cross section schematically illustration another conventional Ch-LCD device. In FIG. 2, a Ch-LCD device 20 includes an upper substrate 21 and a lower substrate 22. Chambers 23 and 24 for liquid crystal flow are interposed between the upper substrate 21 and the lower substrate 22. A cholesteristic liquid crystal layer 26 is disposed in a display area. The gap of the cholesteristic liquid crystal layer 26 is supported by spacers 27. On operation, arrangement of the cholesteristic liquid crystal is changed by exerting a pressure P at the chamber 23. Therefore, it is prevented that the cholesteric liquid crystal is driven to the “H” state under a high voltage. Exerting pressure is not efficient enough to change the entire display frame. Furthermore, it is also impossible to transform the cholesteristic liquid crystal in each pixel area. High reflection losses are still inevitable.
U.S. Pub. No. 2003/0071958, the entirety of which is hereby incorporated by reference, discloses a method for inputting image in a Ch-LCD device using a stylus pressure. FIG. 3 is a cross section schematically showing another conventional Ch-LCD device. A Ch-LCD device 30 includes an upper substrate 37 and a lower substrate 39. Electrodes 38 and 36 are respectively disposed on the upper substrate 37 and the lower substrate 39. An absorption layer 32 is disposed underlying the lower substrate 39. A liquid crystal layer 35 is sandwiched between the upper substrate 37 and the lower substrate 39. The gap of the liquid crystal layer 35 is maintained by spacers 31. Arrangement of the liquid crystal molecules is changed by exerting pressure on the display device using a stylus input device 33. The stylus input cannot efficiently control display gray-scales and switches of display information.
Furthermore, U.S. Pub. No. 2004/0196230, the entirety of which is hereby incorporated by reference, discloses a Ch-LCD system. High voltage is generated by a piezoelectric material under pressure to provided voltage for transforming cholesteric liquid crystal. FIG. 4 is a schematic diagram illustrating a conventional piezoelectric controlled Ch-LCD system. Referring to FIG. 4, a Ch-LCD device 40 includes an upper substrate 44 and a lower substrate 41. Electrodes 45 and 42 are respectively disposed on the upper substrate 44 and the lower substrate 41. An absorption layer 43 is disposed underlying the lower substrate 41. A liquid crystal layer 46 is sandwiched between the upper substrate 44 and the lower substrate 41. The gap of the liquid crystal layer 46 is maintained by spacers 47. A driving device 50 is served to provide high voltage for phase transition of the cholesteric liquid crystal layer 46. The driving device 50 includes a voltage generating device 52 (e.g., a piezoelectric element), a step-down transformer 54, and a phase difference generating circuit 56. Conventional high voltage power is replaced by the piezoelectric element to reduce fabrication cost of the entire system. The reflection losses due to high voltage driving the cholesteric liquid crystal are still inevitable.